This invention relates to signal integrating circuits with a differentiator coupled in a degenerative feedback path for control of the integrator time constant. The integrators are particularly useful in integrated-circuit applications.
Integrating circuits are widely used in electronics for filtering, frequency processing and the like. For example, in television receivers, integrators are used in synchronizing signal separators for separating out low-frequency portions of the composite television signal; in vertical deflection circuits for generating ramp voltages by which the vertical deflection circuit is controlled; in switching regulators as ramp generators for phase modulation; and in automatic frequency control (AFPC) loops for averaging or frequency control. Consumer products such as television receivers are made in large quantities, and cost considerations have caused manufacturers to increasingly use integrated circuits (IC's) in their manufacture.
Advances in integrated-circuit technology have made it possible to concentrate more functions into each IC. The integration of all the television receiver integrated-circuit processes onto a single or few chips is impeded by several factors. As the number of functions performed and therefore the number of semiconductors increases, the power dissipated and therefore the temperature of the device tend to increase. This problem can be solved by increasing the surface area of the device, by heat sinking and the like. Yield limitations prohibit the use of excessively large semiconductor devices.
Each interface between the integrated-circuit semiconductor chip and an external circuit requires a conductor lead, and a bond between the lead and the IC chip. Each such bond adversely affects the yield of operable encapsulated IC's. Also, as the number of functions performed by each chip increases, the number of interface connections also increases until at some point it becomes uneconomical to use a single chip, and the use of multiple chips becomes more advantageous.
The signal integrator is a very common type of circuit. Many television functions and circuits include integrators. While signal integration can be performed with either an inductor or a capacitor, inductors are not commonly used because of their large physical size and because of the electromagnetic fields which they radiate. Commonly, a resistor and a capacitor form the integrator. While both capacitors and resistors can be formed in integrated circuits, inexpensive IC processing techniques typically limit maximum resistances to the order of 10 kilohms, and capacitances to the order of 10 picofarads.
Discrete capacitors having greater capacitance can be connected to the semiconductor chip and enclosed within the chip carrier, but this arrangement is expensive, and the discrete capacitor must be physically large in order to obtain a large capacitance which is stable with temperature. High-K dielectric materials can reduce the physical size of such a discrete capacitor, but the poor temperature coefficient of high-K dielectrics would then make the integrator termperature-sensitive. The conductivity or leakage of high-K dielectrics may be poor. Thus, present day technology does not permit signal integrators with a relatively long time constant (resistance times capacitance) to be formed in an inexpensive manner within the confines of the IC chip.
It is known in principle to increase the effective time constant of an integrator by reducing the charging current to the capacitor, thereby enabling a relatively small charging current to be produced in response to a signal voltage without the use of a large value resistor. However, the sensing circuit by which the integrated voltage across the integrating capacitor is sensed draws a current which cannot easily be predicted or controlled. Thus, when a small integrating current is applied to the integrating capacitor in response to a signal, the capacitor may charge too quickly and therefore exceed the dynamic limits of the following circuits. The capacitor may actually fail to charge if the current required by the sensing circuit exceeds the current being supplied by the signal-responsive current source.
It is known to increase the effective capacitance of an integrator circuit by the Miller integrator configuration. In the Miller integrator, the input current, in principle, flows only to the charging capacitor. However, the Miller integrator includes a sensing amplifier which produces the above-described indeterminate charging current. Furthermore, the Miller integrator, when used as a ramp generator, may result in a decrease, rather than an increase, in the available signal integrating time. Because the voltage at the output of the Miller amplifier changes at a rate established by the time constant of the RC charging circuit external to the amplifier, this rate is maintained for the entire integrating time. As a result, the dynamic range of the Miller amplifier is quickly exceeded, and the Miller action ceases when the amplifier saturates. The Miller action therefore ceases at a time when the voltage across the integrating capacitor is substantially equal to the supply voltage, and this occurs much earlier than it would in the RC integrator. Thus, the Miller integrator trades off signal integrating time for linearity.
Other signal integrating circuits generate ramp waveforms, for example, by coupling a controllable current source to an integrating capacitor. A feedback circuit coupled to the control terminal of the current source regulates the current magnitude. At low current source values, linearity and stability may be degraded.
It is desirable to design a signal integrating circuit in which, for a given resistor and capacitor, the time allowed for signal integration is increased and whereby for a given time constant the integrating capacitor can be made sufficiently small to be conveniently formed on the IC substrate. It is also desirable to design a signal integrator in which leakage of the integrating capacitor does not affect the integrating time whereby lossy dielectrics may be used for forming the integrating capacitor, further aiding in the size reduction. It is further desirable to design a signal integrator in which the linearity of the integrating capacitor does not substantially affect the integrating time, by which it becomes possible to use reverse-biased semiconductor junctions for the integrating capacitor. It is still further desirable to design a signal integrator with a small integrating capacitance and low-value resistors which nevertheless has a relatively long signal integrating time.